1. Field of the Invention
The present invention relates to a toner for electrostatic charged image development which is used when developing an electrostatic latent image formed by electrophotography, electrographic printing or the like with a developer, and also relates to a process for preparing the same, as well as an image forming method, an image forming apparatus and a toner cartridge.
2. Description of the Related Art
A method of visualizing image information via an electrostatic charged image such as electrophotography is currently utilized in a variety of fields. In electrophotography, image information is visualized by forming an electrostatic charged image on a photosensitive body by an electrifying step and an exposing step, developing the electrostatic latent image with a developer containing a toner, and a transferring step and a fixing step.
Known developers include a two-component developer composed of a toner and a carrier, and a one-component developer composed of a magnetic toner or a non-magnetic toner alone. Processes for preparing the toner include a kneading pulverizing method of melting and kneading a thermoplastic resin, a pigment, an electrification controlling agent and a mold releasing agent (such as a wax), cooling, finely-dividing and classifying the mixture is usually utilized. If necessary, in order to improve the flowability and the cleanability, there are cases where fine inorganic particles or fine organic particles are added to the toner particle surfaces. Although these methods provide a considerably excellent toner, they are problematic in certain areas, as described below.
A toner obtained by a normal kneading pulverizing method is undefined in toner shape and surface structure. In the kneading pulverizing method, it is difficult to intentionally control a toner shape and surface structure, although they subtly vary depending on the pulverizability of materials used and the conditions of a pulverizing step. In the kneading pulverizing method, the range of selection of materials is limited. Specifically, in the kneading pulverizing method, the resin and colorant dispersion must be materials that are sufficiently brittle and can be finely-divided with an economical manufacturing apparatus. However, when the resin and colorant dispersion are made to be brittle in order to satisfy this requirement, there are cases when the toner generates a further fine powder, or the toner shape is changed, due to a mechanical shearing force imparted in the developing machine.
Due to these influences, deterioration of electrification in a two-component developer is accelerated due to adhesion of fine powder to the carrier surface. In a one-component developer, toner flight is caused due to expanded particle size distribution, and the image quality is easily deteriorated due to reduction in the developability resulting from changes in the toner shape.
When a toner is prepared by internally adding a large amount of mold releasing agent such as wax, exposure of the mold releasing agent on the surface is caused in the toner in some cases, depending on the combination of the mold releasing agent and thermoplastic resin. In particular, in a combination of a thermoplastic resin which has increased elasticity due to a high molecular component and is slightly difficult to grind, and a wax which is brittle such as polyethylene and polypropylene, exposure of these wax components are observed on the toner surface in many cases. Although this is advantageous in the releasability at fixation and cleaning of untransferred toner from a photosensitive body, since polyethylene in a superficial layer is easily moved by a mechanical force, dirtying of a developing roll, a photosensitive body, and a carrier is easily caused, leading to reduced reliability.
A toner can have insufficient flowability even when a flowing aid is added if the toner shape is undefined. For this reason, movement of a fine particles on the toner surface into concave portions of the toner due to mechanical shearing force during use reduces flowability with time, causes embedment of the flowing aid into the interior of a toner, and deteriorates the developability, transferability and, cleanability. When a toner recovered by cleaning is returned to a developing machine and used, the image quality is further easily reduced. In order to prevent this, when the flowing aid is further increased, black points are generated on the photosensitive body and flight of aid particles occurs.
In recent years, as the means for intentionally controlling toner shape and surface structure, a process for preparing a toner by an emulsion polymerization aggregating method is provided in Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439. This process is generally a process of preparing a toner by making a resin dispersion by emulsion polymerization and, separately, making a colorant dispersion in which a colorant is dispersed in a solvent and, thereafter, mixing them to form an aggregate corresponding to a toner particle diameter, and heating to fuse and coalesce the aggregate.
By this process, a toner shape can be controlled to a degree, and the electrifiability and the durability of a toner can be improved. However, since the internal structure of the toner becomes approximately uniform, there remains a problem on the releasability of a sheet to be fixed upon fixation, and the environment-dependent stability of electrification.
In such electrophotographic processes, in order to stably maintain toner performance even under various mechanical stresses, it is necessary to suppress exposure of the mold releasing agent on the surface, enhance the surface hardness without deteriorating the fixability and, at the same time, improve the mechanical strength of the toner itself, and satisfy the need for both sufficient electrifiability and fixability.
In recent years, there is increased demand for higher image quality and, in image formation, there is a remarkable tendency of miniaturization of a toner in order to realize high-precision imaging. However, with simple miniaturization under the conventional particle size distribution, the presence of fine powder side toner makes dirtying of a carrier and a photosensitive body, as well as toner flight remarkably problematic, and it is difficult to realize both high image quality and high reliability at the same time. For this reason, it is necessary that particle size distribution be sharpened, and miniaturization of a toner be possible.
In addition, from the viewpoint of the recent demand for increasing speed while lowering energy consumption, obtaining uniform electrifiability, durability, toner strength, and sharpness of particle size distribution are becoming increasingly important. Further, in light of increased speed and energy saving, fixability at further lower temperatures becomes necessary. Also from these points, wet processes for preparing toners such as an aggregating coalescent toner, a suspension polymerization toner, and a suspension granulation toner have excellent properties, and wet processes are ideal for providing sharp particle size distribution and for preparing a toner having small particle diameter.
Generally, a polyolefin type wax is internally added to a mold releasing agent component for the purpose of preventing low temperature offset at fixation. In addition, in conjunction with this, there are efforts to improve high temperature offset by uniformly coating a minor amount of silicone oil on the fixing roller. For this reason, silicone oil is adhered on an outputted output transfer receiving material, which is not preferable because when this is handled, the material has a sticky unpleasant feeling.
For this reason, JP-A No. 5-061239 describes a toner for oil-less fixation in which a large amount of mold releasing agent component is internally contained in a toner. However, in this case, although addition of a large amount of mold releasing agent can improve the releasability to an extent, it is difficult to realize stable peeling since compatibility between the binder component and mold releasing agent occurs, and stable exudation of the mold releasing agent is not uniform. Further, since the means for controlling the aggregating force of a binder resin in a toner depends on the Mw and Tg of a binder, it is difficult to directly control the thread-forming property and the aggregating property at fixation of a toner. Further, components freed from a mold releasing agent cause electrification suppression in some cases.
JP-A Nos. 4-69666 and 9-258481 describe a method for solving these problems, providing the method of obtaining inflexibility of a binder resin by addition of a high-molecular component. In addition, JP-A Nos. 59-218460 and 59-218459 describe methods of improving the peelability at oil-less fixation by introducing a chemical cross-linking agent and, as a result, decreasing the thread-forming property at the temperature for toner fixation.
However, when the cross-linking agent component is simply added to a binder as described in JP-A Nos. 59-218460 and 59-218459, since the viscosity of the toner, or the aggregating force at melting becomes great and the inflexibility of the binder resin itself increases, temperature dependency at oil-less peeling and toner mounting amount dependency are improved to an extent, but it is difficult to obtain surface glossiness of a fixed image at the same time.
The bending resistance of a fixed image also becomes deficient. Further, when the molecular weight of the cross-linking agent is merely increased as described in JP-A No. 59-218460, the molecular weight between entanglement points is sure to increase, and the flexibility of the fixed image itself is slightly improved. Nonetheless, it is difficult to obtain suitable balance between the elasticity and the viscosity, and it is difficult to satisfy both of the temperature dependency and the toner mounting amount dependency of peeling at oil-less fixation, not to mention the glossiness of the fixed image surface.
In addition, when a low-temperature low-pressure energy saving-type fixing apparatus is used, or a high-speed copying machine or printer is used, it is fundamentally difficult to obtain a satisfactory fixed image.
JP-A No. 4-69664 describes a method of improving high temperature offset of a toner at fixation with fine polymer particles or fine inorganic particles. When afine inorganic particles are merely added to a toner, toughness at toner binder melting upon fixation is assuredly increased due to the filler effect of the particles, and such a toner exhibits the effect of preventing high temperature offset or improving the peelability. At the same time, the flowability of the melted toner is reduced, and low temperature offset and the glossiness of a fixed image can be deteriorated. Further, the bending resistance of a fixed image is reduced in some cases. In addition, depending on the amount of particles to be added, only the viscosity of the toner at melting is merely increased and, as a result, the peelability is deteriorated in certain cases.
However, in a one-component developer using magnetic metal particles as a colorant, since the specific gravity of the toner can definitely be increased in a melting kneading pulverizing method, which is a dry process, the coloring function and the electrifying function can be suitably controlled. Further, stable electrifiability and coloring property can be manifested at the same time, the system for controlling the toner concentration in the electrophotographic process can be simplified, and an extremely useful toner can be obtained. However, since the controllability of a structure such as a core/shell structure of a toner is deteriorated, there is a problem in flowability, and it is difficult to obtain a precise image.
Meanwhile, in order to solve these problems, new toners and processes are provided, such as an emulsion aggregation and coalescent method (heterogenous aggregating method), a suspension polymerization method, a solubility suspension granulation method, and a solubility emulsion aggregating coalescent method which are wet processes. However, since these wet processes produce a toner particle in an acidic or alkaline aqueous medium, when the fine magnetic metal particles are dispersed in these media, the surface property of the magnetic material itself is greatly changed by oxidation or reduction and, under acidity, the surface of the magnetic material oxidizes, the color changes to a reddish-brown color and, under the alkaline property, iron hydroxide particles are produced, and a change in the magnetism occurs and, therefore, the electrifiability is suppressed.
In addition, under the acidity, a dissolved magnetic particle ion is present in an aqueous medium and, in an emulsion aggregation and coalescent method, since ion balance in an aggregation system is disintegrated, it becomes difficult to control the aggregation rate; in a suspension polymerization system, since polymerization is suppressed, it is particularly difficult to control the particle diameter. Further, in a solubility suspension granulation method and a solubility emulsion aggregating coalescent method, it is difficult to obtain particle stability upon granulation or emulsification.